1. Field of the Invention
The present invention relates to a film formation method using sputtering and a production method of a photovoltaic element using the film formation method, and more particularly to a method of producing a reflective metal layer and a transparent conductive layer stacked thereon by means of a sputtering apparatus.
2. Related Background Art
As the prior art film formation method using sputtering, there is described in, for example, Japanese Patent Application Laid-Open No. 61-64874 a method of forming an oxide film by sputtering which adopts a water vapor concentration of 12% by volume or more and a sputtering gas pressure of 2 mTorr or less to provide a dense, high-quality sputter film and to prevent step-breaking of a wire vaporized on the sputter film.
Further, Japanese Patent Application Laid-Open No. 11-236666 discloses providing a sputtering apparatus mainly composed of a vacuum apparatus with a mass spectrometer and a spectrophotometer, measuring an H2O partial pressure inside the sputter film formation apparatus before and during the film formation, and feeding back the measurement results to adjust the H2O partial pressure, thus constantly forming a dielectric film of a desired dielectric constant.
Moreover, Japanese Patent Application Laid-Open No. 5-171434 describes effecting exhaust of a sputtering chamber to reduce the inner pressure only to 5xc3x9710xe2x88x925 to 5xc3x9710xe2x88x924 Torr to leave some air in the sputtering chamber and depositing nitrogen, oxygen or hydrogen of the remaining air along with sputter particles to provide a high-quality Al-based alloy film.
In addition, Japanese Patent Application Laid-Open No. 6-116722 discloses continuously forming a metal layer and a transparent conductive layer on a long substrate by sputtering while moving the substrate (Roll-to-roll process).
The addition of H2O during sputter film formation in the above mentioned methods provides a deposited film with considerably high quality.
However, in order to mass-produce deposited films with higher quality at a lower cost, there remain the problems to be solved as follows.
For example, the methods described in Japanese Patent Application Laid-Open Nos. 61-64874 and 11-236666 of the above mentioned prior art are techniques accomplished by paying attention to H2O content, and more specifically the method described in Japanese Patent Application Laid-Open No. 61-64874 is accomplished by paying attention to the dielectric constants of dielectric films, but there is no disclosure or suggestion therein of the relationship between deposited films comprised of a metal or metal compound and H2O, especially forming a deposited film comprised of a metal or metal compound while maintaining H2O partial pressure within a predetermined range and applying the deposited film to photovoltaic elements such as solar cells.
Further, the method described in Japanese Patent Application Laid-Open No. 11-236666 is accomplished by paying attention to the shapes of deposited films, but there is no disclosure or suggestion therein of optical characteristics represented by reflectance or the like of deposited films.
Moreover, Japanese Patent Application Laid-Open No. 5-171434 has no description of specific H2O content, and use of only the H2O content controlling method defined therein may lead to variability of characteristics of deposited films due to a large variance of H2O content resulting from, e.g., a change in the temperature and humidity of an atmosphere during maintenance and a change in the surface area or material of the inner wall of a film formation chamber.
Further, although the problem in long-time film formation using a long substrate by, e.g., the roll-to-roll process is to constantly maintain characteristics, there is no description in the above mentioned patent gazettes of characteristic stability during long-time film formation by the roll-to-roll process and application of deposited films to photovoltaic elements. That is, with long-time film formation by the roll-to-roll process, only rear surface reflective layers with large variability of characteristics such as reflectance or the like of rear surface reflective films can be formed due to change in the film forming atmosphere and the target surface state. For example, when the target material is a low-melting metal such as In, there is a case where the rise in temperature of the target during the film formation may melt a part of a surface of the target to vary the sputtering efficiency. Photovoltaic elements obtained by stacking a photoelectric conversion layer and a transparent electrode layer using such a rear surface reflective layer with large variability of characteristics will have large variability of photoelectric conversion efficiency and give rise to lowering in photoelectric conversion efficiency. Further, applying a pulling force of about 15000 xcexcxcex5 to such a photovoltaic element will generate a number of cracks therein, and an increase in resistance due to extension of a wiring route will lower photoelectric conversion efficiency, so that workability and durability may be lowered.
It is, therefore, an object of the present invention to provide a film formation method using sputtering and a production method of a photovoltaic element using the film formation method that solve the above mentioned problems of the prior art, attain stable good film formation even during long-time sputter film formation, can constantly form a reflective film with a desired reflectance, has excellent workability and durability, and constantly attain high photoelectric conversion efficiency.
In order to attain the above mentioned object, the present invention provides a film formation method using sputtering and a production method of a photovoltaic element using the film formation method featured by (1)-(17) below.
(1) A film formation method using sputtering which comprises introducing a sputtering gas into a film forming chamber and forming a film on a substrate therein, wherein the partial pressure of H2O in an atmosphere inside the film forming chamber is controlled so as to satisfy prescribed conditions, thereby forming a reflective layer of a prescribed reflectance on the substrate.
(2) The method as mentioned in (1) above, wherein the partial pressure of H2O in the atmosphere is controlled so as to be not less than 6.6xc3x9710xe2x88x924 Pa but no more than 1.4xc3x9710xe2x88x922 Pa.
(3) The method as mentioned in (1) above, wherein during the film formation, the partial pressures of H2O at the beginning and completion of the film formation are each controlled so as to be not less than 6.6xc3x9710xe2x88x924 Pa but no more than 1.4xc3x9710xe2x88x922 Pa.
(4) The method as mentioned in (1) above, wherein during the film formation, the partial pressures of H2O at the beginning and completion of the film formation are each controlled so as to be not less than 2.6xc3x9710xe2x88x923 Pa but no more than 6.7xc3x9710xe2x88x923 Pa.
(5) The method as mentioned in any one of (1)-(4) above, wherein the partial pressure of H2O is detected during the film formation.
(6) The method as mentioned in (5) above, wherein the partial pressure of H2O is controlled based on the result of the detection.
(7) The method as mentioned in (6) above, wherein the partial pressure of H2O is controlled by supplying H2O into the film forming chamber and adjusting the concentration of the H2O.
(8) The method as mentioned in any one of (5)-(7) above, wherein the detection is effected using a tetrode mass spectrometer.
(9) The method as mentioned in any one of (1)-(8) above, wherein prior to the film formation, the interior of the film forming chamber is baked.
(10) The method as mentioned in any one of (1)-(9) above, wherein the film comprises a metal.
(11) The method as mentioned in (10) above, wherein the metal is Al, Ag or Cu.
(12) The method as mentioned in any one of (1)-(9) above, wherein the film comprises a metal oxide.
(13) The method as mentioned in (12) above, wherein the metal oxide is a metal oxide containing at least one selected from Al, In, Sn, Zn, and Ti.
(14) The method as mentioned in any one of (1)-(13) above, wherein the substrate is a belt-like substrate, and the film is formed on the belt-like substrate while successively moving the belt-like substrate.
(15) A method of producing a photovoltaic element comprising a rear surface reflective layer, a photoelectric conversion layer and a transparent electrode layer on a substrate, which comprises forming the rear surface reflective layer using the film formation method as mentioned in any one of (1)-(14) above.
(16) A method of producing a photovoltaic element comprising a transparent conductive layer, a photoelectric conversion layer and a transparent electrode layer on a substrate, which comprises forming the transparent conductive layer using the film formation method as mentioned in any one of (1)-(14) above.
(17) A method of producing a photovoltaic element comprising a rear surface reflective layer, a transparent conductive layer, a photoelectric conversion layer and a transparent electrode layer on a substrate, which comprises forming at least one of the rear surface reflective layer and the transparent conductive layer using the film formation method as mentioned in any one of (1)-(14) above.